A known technique for providing an efficient means of power amplifying a non-constant envelope signal is to use envelope modulation. Envelope modulation is typically provided by means of a highly efficient modulatable power supply.
However, in such an arrangement the power amplifier presents an impedance to the modulatable supply. In particular, this impedance can change rapidly with frequency due to a presence of a high Q series resonance, making accurate high bandwidth modulation very difficult. In a typical transistor output stage, the transistor device capacitance resonates with the inductance of the supply feed to form such a series resonance. This series resonance presents problems if the supply modulation is to be applied at high bandwidths.
One solution is to redesign the power output stage to move the resonance to a higher frequency. However it is difficult to achieve this without compromising the output stage with respect to some other parameter. For example reducing the capacitance can only be achieved by changing the output device technology from, for example, LDMOS to GaAs mesfet or Phemt. This could potentially have a major implication on the cost of such units, as well as the ability to manufacture such units. Reducing the inductance of the supply feed, as an alternative, would affect the load presented to the transistor at RF and its harmonics. This could compromise the RF stage efficiency, gain or bandwidth.
Thus a problem exists with current power amplifier output stages in that at high frequencies a series resonance is presented at the power supply terminal of the amplifier as a consequence of the output capacitance of the transistor forming a high Q resonant network with the supply feed inductance. This network creates a source of potential instability, and creates a long time constant energy storage means that contributes to memory effect, and renders any form of wideband envelope modulation difficult to apply.
It is an aim of the present invention to provide an improved technique which addresses one or more of the above stated problems.